In a closed cooling system filled with a coolant, temperature changes as well as permeation, for example through tube walls, result in a change in the volume of the coolant. Some compensation or equalization for this coolant volume change, that ensures that no, or only small, pressure changes occur in the system, must be found.
Such changes in volume can be buffered by means of a so-called equalizing vessel. This causes additional costs, however, and also increases the risk of cooling medium leaking out.
An important problem in the context of heat exchangers for electronic devices is that their exact operating orientation is not known, a priori. This is true not least for transportation to the customer, since such cooling systems are already filled with cooling medium at the manufacturer's premises, and the orientation they will assume during transport cannot be predicted. The same is true for utilization in vehicles of all kinds (aircraft, ships, land vehicles, vehicles in a weightless state). Operating reliability must therefore be guaranteed in all conceivable operating orientations. If liquid were to mix with gas in the cooling circuit, reliable operation of a circulating pump would then no longer be guaranteed, with the result that cooling performance might rapidly decrease. This would then very quickly cause the electronic component being cooled either to switch itself off, or to be destroyed by the increase in temperature.